Download The End of Nature versus Nurture

The End of
Nature versus
Nurture
THOMAS WANSTALL The Image Works
by Frans B. M. de Waal
Twins reared apart
have been studied
for clues about the
relative contributions of genes and
environment to
human behavior.
These brothers
rediscovered each
other later in life
when both were
mustachioed
firefighters.
94
Copyright 1999 Scientific American, Inc.
Is human behavior determined by genetics or by environment?
It may be time to abandon the dichotomy
T
he defenders of nature and nurture have been at each other’s throats for as long as I can remember. Whereas biologists have always believed that genes have something to do with human behavior, social scientists have flocked en masse to the opposite position: that we are fully and entirely our own creation, free from the chains of biology.
I felt the heat of this debate in the 1970s whenever, in lectures for general audiences, I mentioned sex dif-
ferences in chimpanzees, such as that males are more aggressive and more ambitious than females. There
Copyright 1999 Scientific American, Inc.
would be howls of protest. Wasn’t I projecting
my own values onto these poor animals? How
rigorous were my methods? Why did I even
bother to compare the sexes? Did I perhaps have
a hidden agenda?
Nowadays the same sort of information makes
people yawn! Even direct comparisons between
human and ape behavior, something that used to
be taboo, fail to get anyone excited. Everyone has
heard that men are from Mars and women from
Venus. Everyone has seen, in Time and Newsweek,
PET scans of the human brain engaged in various
tasks, with different areas lighting up in male and
female brains.
This time, however, it is my turn to be troubled.
Instead of celebrating the victory of the biological
approach, I regard some of the contemporary dichotomies between men and women as gross simplifications rendered politically correct by a fashionable amount of male-bashing (for example,
when normal hormonal effects are referred to as
“testosterone poisoning”). We remain as far removed as ever from a sophisticated understanding
of the interplay between genes and environment.
Society has let the pendulum swing wildly back
from nurture to nature, leaving behind a number
of bewildered social scientists. Yet we still love to
phrase everything in terms of one influence or the
other, rather than both.
It is impossible to explore where we may be
heading 50 years from now without looking back
an equal number of years at the charged history
of the nature/nurture controversy. The debate is
so emotional because any stance one takes comes
with serious political implications. Positions have
ranged from an unfounded faith in human flexi96
SCIENTIFIC AMERICAN
SOVFOTO/EASTFOTO
CORBIS
Danger comes from
extremes of both
positions— the biological
determinism of the Nazis
and the social engineering of the Communists.
bility by reformists to an obsession with blood
and race by conservatives. Each in their own way,
these positions have caused incalculable human
suffering in the past century.
Learning and Instinct
F
ifty years ago the two dominant schools of
thought about animal and human behavior
had opposite outlooks. Teaching animals arbitrary actions such as lever-pressing, American behaviorists came to view all behavior as the product of trial-and-error learning. This process was
considered so universal that differences among
species were irrelevant: learning applied to all animals, including humans. As B. F. Skinner, the
founder of behaviorism, bluntly put it: “Pigeon,
rat, monkey, which is which? It doesn’t matter.”
In contrast, the ethological school in Europe
focused on naturalistic behavior. Each animal
species is born with a number of so-called fixedaction patterns that undergo little modification
by the environment. These and other speciesspecific behaviors represent evolutionary adaptations. Thus, no one needs to teach humans how
to laugh or cry: these are innate signals, universally used and understood. Similarly, the spider
does not need to learn how to construct a web.
She is born with a battery of spinnerets (spinning
tubes connected to silk glands) as well as a behavioral program that “instructs” her how to weave
threads together.
Because of their simplicity, both views of behavior had enormous appeal. And although both
paid homage to evolution, they sometimes did so
in a superficial, arm-waving sort of way. Behav-
December 1999
The End of Nature versus Nurture
Copyright 1999 Scientific American, Inc.
iorists stressed the continuities between humans
and other animals, attributing these to evolution.
But because for them behavior was learned rather
than inborn,they ignored the genetic side, which
is really what evolution is all about. While it is
true that evolution implies continuity, it also implies diversity: each animal is adapted to a specific way of life in a specific environment. As is evident from Skinner’s statement, this point was
blithely ignored.
Similarly, some ethologists had rather vague
evolutionary notions, emphasizing phylogenetic
descent rather than the processes of natural selection. They saw behavioral traits, such as the inhibition of aggression, as good for the species. The
argument was that if animals were to kill one another in fights, the species would not survive.
This may be true, but animals have perfectly
selfish reasons to avoid the escalation of fights
that may harm themselves and their relationships. Hence, these ideas have now been replaced
by theories about how traits benefit the actor and
its kin; effects on the species as a whole are considered a mere by-product.
Behaviorism started losing its grip with the discovery that learning is not the same for all situations and all species. For example, a rat normally
links actions with effects only if the two immediately follow each other. So it would be very slow
to learn to press a bar if a reward followed minutes later. When it comes to food that makes it
sick, however, a delay of hours between consumption and the negative sensation still induces future food aversion. Apparently, animals
are specialized learners, being best at those contingencies that are most important for survival.
At the same time that behaviorists were forced
to adopt the premises of evolutionary biology and
to consider the world outside the laboratory, ethologists and ecologists were laying the groundwork
for the neo-Darwinian revolution of the 1970s.
The pioneer here was Dutch ethologist Nikolaas
Tinbergen, who conducted ingenious field experiments on the survival value of animal behavior.
He understood, for instance, why many birds remove eggshells from the nest after the chicks have
hatched. Because the outside of a shell is colored
for camouflage but the inside is not, predators
such as crows easily locate eggs if broken shells are
placed next to them. Throwing out the pieces is an
automatic response favored by natural selection
because the birds that practice this behavior have
more surviving offspring.
Others developed theories to explain behavior
that at first sight does not seem to help the actor
but someone else. Such “altruism” can be seen in
ant soldiers giving their lives in defense of their
colony or in dolphins lifting a drowning companion to the surface. Biologists assumed that natural
selection will allow for assistance among relatives
as a means of promoting the same genes. Or, if
two animals are unrelated, the favor granted by
one must be returned at some future time.
The End of Nature versus Nurture
The scientists felt so confident about their explanations of cooperative animal societies that
they could not resist extending these ideas to our
own species. They saw the hugely cooperative
enterprise of human society as based on the same
premise of family values and economic tit-for-tat.
It fell to an American expert on ants, Edward O.
Wilson, to deliver the news in 1975 that a great
deal of human behavior was ripe for the Darwinian perspective and that the social sciences should
prepare themselves to work together with biologists on this endeavor. Thus far the two disciplines
had led separate lives, but from the perspective of
a biologist social science is not much more than
the study of animal behavior focused on a single
species: ours. Because this is not how social scientists see their work, proposals for a united framework were not kindly received. One of Wilson’s
outraged opponents even poured cold water over
Wilson’s head after he gave a lecture. For reasons
explained below, his new synthesis, dubbed “sociobiology,” was equated with race policies of the
past and ultimately with the Holocaust.
Although the criticism was patently unfair—
Wilson was offering evolutionary explanations,
not policy suggestions—we shouldn’t be surprised that the topic of human biology arouses
strong emotions.
Burdens of the Past
I
t is generally believed that some human behavior can easily be changed because it is learned,
whereas other behavior resists modification because it is part of our biological heritage.
Ideologues of all colors have grasped this division to argue for the innate nature of certain human characteristics (for example, purported race
differences in intelligence) and the plasticity of others (such as the ability to overcome gender stereotypes). Thus, Communism was founded on great
confidence in human malleability. Because people,
unlike social insects, resist submerging individuality for the greater good, some regimes accompanied
their revolutions with massive indoctrination efforts. All of this proved in vain, however. Communism went under because of an economic incentive structure that was out of touch with human
nature. Unfortunately, it did so only after having
caused great misery and death.
Even more disastrous was the embrace of biology by Nazi Germany. Here, too, the collective (das
Volk) was placed above the individual, but instead
of relying on social engineering the method of
choice was genetic manipulation. People were
classified into “superior” and “inferior” types, the
first of which needed to be protected against contamination by the second. In the horrible medical
language of the Nazis, a healthy Volk required the
cutting out of all “cancerous” elements. This idea
was followed to its extreme in a manner that
Western civilization has vowed never to forget.
Don’t think that the underlying selectionist
SCIENTIFIC AMERICAN
Copyright 1999 Scientific American, Inc.
December 1999
97
FRANS LANTING Minden Pictures
Our closest animal
relatives— such as this
bonobo family— share
many human behaviors. Television nature
programs have brought
home to the general
public the lesson of
biology’s influence
on behavior.
98
SCIENTIFIC AMERICAN
ideology was restricted to this particular time and
place, however. In the early part of the 20th century, the eugenics movement— which sought to
improve humanity by “breeding from the fitter
stocks”— enjoyed widespread appeal among intellectuals in both the U.S. and Great Britain.
Based on ideas going back to Plato’s Republic, sterilization of the mentally handicapped and of
criminals was considered perfectly acceptable.
And social Darwinism— the idea that in a laissezfaire capitalist economy the strong will outcompete the weak, resulting in general improvement
of the population— still inspires political agendas
today. In this view, the poor should not be aided
in their struggle for existence so as not to upset
the natural order.
Given these ideologies, it is understandable why suppressed
categories of people,
such as minorities and
women, fail to see biology as a friend. I would
argue, however, that
the danger comes from
both directions, from
biological determinism
as well as its opposite,
the denial of basic human needs and the belief that we can be everything we want to
be. The hippie communes of the 1960s, the Israeli kibbutzim and the feminist revolution all
sought to redefine humans. But denial of sexual
jealousy, the parent-child bond or gender differences can be carried only so far before a countermovement will seek to balance cultural trends
with evolved human inclinations.
What makes the present era different is that
the genocide of World War II is fading into memory while at the same time the evidence for a
connection between genes and behavior is
mounting. Studies of twins reared apart have
reached the status of common knowledge, and
almost every week newspapers report a new human gene. There is evidence for genes involved
in schizophrenia, epilepsy and Alzheimer’s and
even in common behavioral traits such as thrillseeking. We are also learning more about genetic
and neurological differences between men and
women, as well as between gay and straight men.
For example, a small region of the brain in transsexual men (who dress and behave like women)
resembles the same region in women’s brains.
The list of such scientific advances is getting
longer by the day, resulting in a critical mass of
evidence that is impossible to ignore. Understandably, academics who have spent their life
condemning the idea that biology influences human behavior are reluctant to change course. But
they are being overtaken by the general public,
which seems to have accepted that genes are in-
volved in just about everything we do and are.
Concurrently resistance to comparisons with
other animals has dissipated because of a stream
of television nature programs that has brought
exotic wildlife into our homes while showing animals to be quite a bit smarter and more interesting than people used to believe.
Studies of chimpanzees and bonobos, such as
those by Jane Goodall and myself, show that
countless human practices and potentials, from
politics and child-rearing to violence and even
morality, have parallels in the lives of our closest
animal relatives. How can we maintain the dualisms of the past—between humans and animals
and between body and mind—in the face of all
this evidence to the contrary? Current knowledge
about our biological background simply doesn’t
permit a return to the tabula rasa views of the past.
This doesn’t solve the problem of ideological
abuse, however. If anything, it makes things
worse. So long as people have political agendas,
they will depict human nature one way or another for their own purposes. Conservatives like to
point out that people are naturally selfish, whereas liberals argue that we have evolved to be social
and cooperative. The obvious correctness of both
inferences goes to show what is wrong with simple-minded genetic determinism.
The Best of Both Worlds
B
ecause genetic language (“a gene for x”) plays
into our sound-bite culture, there is all the
more reason to educate the public that genes, by
themselves, are like seeds dropped onto the pavement: powerless to produce anything. When scientists say that a trait is inherited, all they mean
is that part of its variability is explained by genetic factors. That the environment usually explains
at least as much tends to be forgotten.
As Hans Kummer, a Swiss primatologist, remarked years ago, to try to determine how much
of a trait is produced by genes and how much by
the environment is as useless as asking whether
the drumming that we hear in the distance is
made by the percussionist or by his instrument.
On the other hand, if we pick up distinct sounds
on different occasions, we can legitimately ask
whether the variation is caused by different
drummers or by different drums. This is the only
sort of question science addresses when it looks
into genetic versus environmental effects.
I foresee a continued mapping of the links between genes and behavior, a much more precise
knowledge of how the brain works and a gradual
adoption of the evolutionary paradigm in the social sciences. Charles Darwin’s portrait will finally decorate the walls of departments of psychology and sociology! But one would hope that all of
this will be accompanied by continued assessment of the ethical and political implications of
behavioral science.
Traditionally, scientists have acted as if it is none
December 1999
The End of Nature versus Nurture
Copyright 1999 Scientific American, Inc.
The End of Nature versus Nurture
tive primates on the earth— this is the one partner
combination in which sex is extremely rare or absent. Incest avoidance has now been convincingly
demonstrated in a host of primates, and the mediating mechanism is thought to be early familiarity.
The Westermarck effect serves as a showcase
for Darwinian approaches to human behavior
because it so clearly rests on a combination of nature and nurture. The framework includes a developmental component (learned sexual aversion), an innate component (the effect of early
familiarity), a cultural component (some cultures
raise unrelated children together, others raise siblings of the opposite sex apart, but most have
family arrangements that automatically lead to
sexual inhibitions among relatives), a sound evolutionary reason (suppression of inbreeding) and
direct parallels with animal behavior. On top of
this comes the cultural taboo, which is unique to
our species. An intriguing question is whether
the incest taboo merely serves to formalize and
strengthen the Westermarck effect or whether it
adds a substantially new dimension.
The unexpected richness of a research program
that integrates developmental, genetic, evolutionary and cultural approaches to a well-circumscribed phenomenon demonstrates the power of
breaking down old barriers between disciplines.
Most likely what will happen in the next millennium is that evolutionary approaches to human
behavior will become more and more sophisticated by explicitly taking cultural flexibility into
account. Hence, the traditional either/or approach to learning and instinct will be replaced
by a more integrated perspective. In the meantime, students of animal behavior will become
more interested in environmental effects on behavior and especially—in animals such as primates and marine mammals—the possibility of
cultural transmission of information and habits.
For example, some chimpanzee communities use
stones to crack nuts in the forest, whereas other
communities have the same nuts and stones
available but don’t do anything with them. Such
differences are unexplained by genetic variation.
These two developments together will weaken
the dichotomies popular today to the point of
eliminating them. Rather than looking at culture
as the antithesis of nature, we will be gaining a
much more profound understanding of human
behavior by silently carrying the old nature/nurture debate to its grave.
SA
The Author
CATHERINE MARIN
of their business how the information they produce is being used. During some periods they have
even actively assisted in political abuse. One notable exception was, of course, Albert Einstein, who
may serve as a model of the kind of moral awareness needed in the behavioral and social sciences. If
history teaches us anything, it is that it is critical
that we remain on the alert against misinterpretations and simplifications. No one is in a better position than the scientists themselves to warn against
distortions and to explain the complexities.
In which direction the thinking may develop
can perhaps be illustrated with an example from
the crossroads between cultural and evolutionary
anthropology. Sigmund Freud and many traditional anthropologists, such as Claude LéviStrauss, have assumed that the human incest
taboo serves to suppress sexual urges between
family members. Freud believed that “the earliest
sexual excitations of youthful human beings are
invariably of an incestuous character.” Hence,
the incest taboo was seen as the ultimate victory
of culture over nature.
In contrast, Edward Westermarck, a Finnish sociologist who lived at about the same time as
Freud, hypothesized that early familiarity (such
as between mother and child and between siblings) kills sexual desire. Little or no sexual attraction is found, he argued, between individuals
who have grown up together. A fervent Darwinian, Westermarck proposed this as an evolved
mechanism designed to prevent the deleterious
consequences of inbreeding.
In the largest-scale study on this issue to date,
Arthur P. Wolf, an anthropologist at Stanford University, examined the marital histories of 14,400
women in a “natural experiment” carried out in
Taiwan. Families in this region used to adopt and
raise future daughters-in-law, which meant that
intended marriage partners grew up together from
early childhood. Wolf compared these marriages
with those arranged between men and women
who did not meet until the wedding day. Using divorce and fertility rates as gauges of marital happiness and sexual activity, respectively, the data
strongly supported the Westermarck effect: association in the first years of life appeared to compromise adult marital compatibility. Nonhuman primates are subject to the same mechanism. Many
primates prevent inbreeding through migration of
one sex or the other at puberty. The migratory sex
meets new, unrelated mates, whereas the resident
sex gains genetic diversity from the outside. But
close kin who stay together also generally avoid
sexual intercourse.
Kisaburo Tokuda first observed this in a group of
Japanese macaques at the Kyoto zoo in the 1950s.
A young adult male that had risen to the top rank
made full use of his sexual privileges, mating frequently with all the females except for one: his
mother. This was not an isolated case: mother-son
matings are strongly suppressed in all primates.
Even in bonobos— probably the most sexually ac-
FRANS B. M. DE WAAL
was trained as a zoologist
and ethologist in the European tradition in his native country, the Netherlands. He has been in the
U.S. since 1981 and is currently director of the Living Links Center at the
Yerkes Regional Primate
Research Center in
Atlanta and is also C. H.
Candler Professor of
Primate Behavior in the
psychology department
at Emory University. His
research includes social
interactions in primates
as well as the origins
of morality and justice
in human society.
Further Information
SOCIOBIOLOGY: THE NEW SYNTHESIS. EDWARD O. WILSON. Belknap Press (Harvard
University Press), 1975. 25th anniversary edition (in press).
SEXUAL ATTRACTION AND CHILDHOOD ASSOCIATION: A CHINESE BRIEF FOR EDWARD
WESTERMARCK. Arthur P. Wolf. Stanford University Press, 1995.
THE MISMEASURE OF MAN. Revised edition. Stephen Jay Gould. W. W. Norton,
1996.
GOOD NATURED: THE ORIGINS OF RIGHT AND WRONG IN HUMANS AND OTHER ANIMALS. Frans de Waal. Harvard University Press, 1997.
SCIENTIFIC AMERICAN
Copyright 1999 Scientific American, Inc.
December 1999
99